4 Answers
4

These two games use different engines. I was going to write some differences that I remember, but then I found a set of videos on YouTube that compare a lot of things between CS 1.6 and CS:S. Here's the final result of these videos:

A short explanation of every point:

Grenade Sizes: they are bigger in 1.6. So it's somewhat easier to see them and get a chance to evade them

Grenade Physics: grenades in 1.6 fly where you throw them. They are predictable and if they bounce from the wall you do know where will they land(more or less)

Player-World Scale: CS:S looks smaller. On dust2 you can just jump on the higher part that is on B, the boxes there are headshot-boxes and not a stack of 2 anymore. And these low boxes give an advantage to the Terrorists.

Movement: pretty much the same, but in CS:S if you move in one direction and then you immediately move to the other one there is a tiny lag.

No Barrels & Junk: that is self-explanatory. If you played CS:S you know that these objects are extremely annoying.

Weapons: overpowered M4 in CS:S and underpowered AK47. More info is in the video.

Wall Spamming: shooting through the walls. In CS:S you can only shoot through tiny walls and/or doors, in 1.6 you can shoot almost through everything(if you played fy_iceworld in 1.5 of 1.6 you should know that people can easily get a kill through 2 huge iceblocks with an AWP at the very beginning of a round)

Smokes: smoke from an SG is thicker and it appears not immediately. You can't see through it and 1 SG is enough to obscure the visibility. In 1.6 you needed 2 SGs to make the smoke solid.

Grenades (HE): the difference is that in CS:S the grenades do not hurt people standing behind the wall. In 1.6 if you would throw a grenade to the wall and an enemy would stand just behind the wall you could possible kill him.

The main difference is the way you use weapons. For example weapons shoot differently, they tend to spray a bit more, causing a loss of accuracy. Grenades are not thrown the same way (there is a different gravity). Maps have been remodeled so there's some box or barrel added to add some more safe spots. There isn't much more mechanics to the game, it still is the same game.

I no longer have CS 1.6 but I still got Condition Zero, which is, in my opinion, plays identical to 1.6, except for the overpowered spread weapons (HE, Shotguns, Machine Gun).

Here are the differences that I've observed (some may be already mentioned):

Weapons:

CS:S has approximately 25% less recoil compare to CS:CZ, but is more sensitive to movement, especially the AWP (Arctic Warfare Police) sniper rifle. Tapping the walk key as you fire your weapon really helps.

The spray pattern is not that consistent.

The quick-shot (I think it's an anomaly) trick with a sniper rifle is no longer available.

Crouching/ducking now only gives a little accuracy boost. As in CS:CZ, it works like a charm.

The 2nd shot with the Desert Eagle is no longer magically accurate.

Weapon damage is now variable, such as: in a very close range, an MP5 can give a killer 100 damage to the head if your opponent is not wearing a helmet.

You will no longer be pushed slightly if you got hit at the body while standing or in mid-air.

Ammunition is now free (which I personally do not like) and your weapons will be automatically reloaded at the start of the next round. In CS:CZ you need to at least hit the reload key.

You can now only wall-bang though thin walls.

Grenades:

Grenades have different physics that is dependent on your movement. And the way you throw it is now harder, but it bounces less.

Flashbang has a different overall effect.

HE-Grenade does less damage and that damage is block-able by a wall.

Both flashbang and he-grenade could jam your hearing for a while if it explodes near you.

The effect of smoke grenade is really thick.

The physical size of grenades are now smaller.

Movement:

It is impossible to make a quick turn from walking in one direction for a while. There is a noticeable delay as if your character is having a hard time gathering a momentum. I really hate it.

Misc:

Falling from a height now gives a variable damage, depending on the weapon you are carrying.

The sound effects of weapon firing, reloading, and picking equipments are now louder.

The initial sound of defusing a bomb has a loudness that is dependent on your range to the bomb location.

The footsteps of your team mates and opponents are now much harder to hear.

The head hitbox anomaly while planting/defusing a bomb has been removed, but it seems that it made a comeback with the orange box update.

The crosshair has a different scale but shows how the weapon is sensitive to movement. If you want to restore the classic one, you must enter these console commands: [cl_dynamiccrosshair 2; cl_legacy_crosshair 1; cl_legacy_crosshair_recoil 1]

Custom crosshair color and center dot is now available.

Body armor is now tougher. It's best to go for the head shot whenever possible.

The expert bots are now lethal. And whatever happens, don't let them carry an auto sniper (especially the G3SG1) because, ooooooohh boy!! XD